Networking of differentially expressed genes in human cancer cells resistant to methotrexate
© Selga et al.; licensee BioMed Central Ltd. 2009
Received: 22 May 2009
Accepted: 4 September 2009
Published: 4 September 2009
The need for an integrated view of data obtained from high-throughput technologies gave rise to network analyses. These are especially useful to rationalize how external perturbations propagate through the expression of genes. To address this issue in the case of drug resistance, we constructed biological association networks of genes differentially expressed in cell lines resistant to methotrexate (MTX).
Seven cell lines representative of different types of cancer, including colon cancer (HT29 and Caco2), breast cancer (MCF-7 and MDA-MB-468), pancreatic cancer (MIA PaCa-2), erythroblastic leukemia (K562) and osteosarcoma (Saos-2), were used. The differential expression pattern between sensitive and MTX-resistant cells was determined by whole human genome microarrays and analyzed with the GeneSpring GX software package. Genes deregulated in common between the different cancer cell lines served to generate biological association networks using the Pathway Architect software.
Dikkopf homolog-1 (DKK1) is a highly interconnected node in the network generated with genes in common between the two colon cancer cell lines, and functional validations of this target using small interfering RNAs (siRNAs) showed a chemosensitization toward MTX. Members of the UDP-glucuronosyltransferase 1A (UGT1A) family formed a network of genes differentially expressed in the two breast cancer cell lines. siRNA treatment against UGT1A also showed an increase in MTX sensitivity. Eukaryotic translation elongation factor 1 alpha 1 (EEF1A1) was overexpressed among the pancreatic cancer, leukemia and osteosarcoma cell lines, and siRNA treatment against EEF1A1 produced a chemosensitization toward MTX.
Biological association networks identified DKK1, UGT1A s and EEF1A1 as important gene nodes in MTX-resistance. Treatments using siRNA technology against these three genes showed chemosensitization toward MTX.
The large amount of information obtained with high-throughput technologies like expression microarrays needs to be processed in order to be comprehensible to molecular biologists. In this regard, many computational methods have been developed to facilitate expression data analysis. Gene clustering, gene ontology and pathway analyses are commonly used [1, 2]. Pathways are manually generated diagrams that represent knowledge on molecular interactions and reactions  and they can be used to visualize the involvement of the differentially expressed genes in specific molecular, cellular or biological processes. However, the complexity of higher organisms cannot be explained solely as a collection of separate parts ; in organisms, pathways never exist in isolation, they are part of larger networks, which are more informative and real . Gene networks are capable of describing a large number of interactions in a concise way, and provide a view of the physiological state of an organism at the mRNA level. Biochemical networks can be constructed at several levels and can represent different types of interactions. Literature mining allows the extraction of meaningful biological information from publications to generate networks . Taking into account the progress in gene expression profiling, elucidating gene networks is an appropriate and timely step on the way to uncovering the complete biochemical networks of cells .
In this work, we use biological association networks (BANs) as a tool to define possible targets for gene therapy in combination with methotrexate (MTX). This approach could serve to minimize the development of MTX resistance acquired by cancer cells, which remains a primary cause of therapy failure in cancer treatment . A role in MTX resistance was established for the three node genes selected, namely those encoding Dikkopf homolg 1 (DKK1), UDP-glucuronosyltransferases (UGTs; UGT1A s) and Eukaryotic translation elongation factor 1A1 (EEF1A1).
Cell lines representative of five types of human cancer were used: HT29 and Caco-2 for colon cancer, MCF-7 and MDA-MB-468 for breast cancer, MIA PaCa-2 for pancreatic cancer, K562 for erythroblastic leukemia, and Saos-2 for osteosarcoma. These cell lines are sensitive to MTX, with IC50s of 1.67 × 10-8 M MTX for HT29, 4.87 × 10-8 M MTX for MDA-MB-468 and 1.16 × 10-8 M MTX for MIA PaCa-2 cells. IC50 values were calculated using GraphPad Prism 5 version 5.0a for Macintosh (GraphPad Software, San Diego, CA, USA). Resistant cells were obtained in the laboratory upon incubation with stepwise concentrations of MTX (Lederle) as previously described . HT29, Caco-2 and K562 resistant cells were able to grow in 10-5 M MTX; MIA PaCa-2, Saos-2, MCF-7 and MDA-MB-246 cells were resistant to 10-6 M MTX.
Human cell lines were routinely grown in Ham's F12 medium supplemented with 7% fetal bovine serum (both from Gibco/Invitrogen, Grand Island, NY, USA) at 37°C in a 5% CO2 humidified atmosphere. Resistant cells were routinely grown in selective DHFR medium lacking glycine, hypoxanthine and thymidine (-GHT medium; Gibco), the final products of dihydrofolate reductase (DHFR) activity. This medium was supplemented with 7% dialyzed fetal bovine serum (Gibco).
Gene expression was analyzed by hybridization to the GeneChip® Human Genome U133 PLUS 2.0 from Affymetrix, containing over 54,000 transcripts and variants. Total RNA for oligo arrays was prepared from triplicate samples of every sensitive and resistant cell line using the RNAeasy Mini kit (Qiagen, Germantown, Maryland, USA) following the recommendations of the manufacturer. Labeling, hybridization and detection were carried out following the manufacturer's specifications.
Microarray data analyses
Gene expression analyses were performed using three samples of both sensitive and resistant cells for each of the seven cell lines studied. These analyses were carried out with the GeneSpring GX software v 7.3.1 (Agilent Technologies, Santa Clara, CA, USA), using the latest gene annotations available (March 2009). This software package allows multi-filter comparisons using data from different experiments to perform the normalization, generation of restriction (filtered) lists and functional classifications of the differentially expressed genes. Normalization was applied in two steps: 'per chip normalization', by which each measurement was divided by the 50th percentile of all measurements in its array; and 'per gene normalization', by which all the samples were normalized against the median of the control samples (sensitive cells). The expression of each gene was reported as the ratio of the value obtained for each condition relative to the control condition after normalization of the data. Then, data were filtered using the control strength, a control value calculated using the Cross-Gene Error model on replicates  and based on average base/proportional value. Measurements with higher control strength are relatively more precise than measurements with lower control strength. Genes that did not reach this value were discarded. Additional filtering was performed to determine differentially expressed genes. A first filter was performed by selecting the genes that displayed a P-value corrected by false discovery rate (Benjamini and Hochberg false discovery rate) of less than 0.05. The output of this analysis was then filtered by fold expression. Thus, lists of genes differentially expressed by at least twofold were generated for each of the seven resistant cell lines.
Common genes between cell lines
The lists of genes differentially expressed by at least twofold with a P-value < 0.05 including multiple testing correction for each cell line were divided into two groups: overexpressed and underexpressed genes. Comparisons of lists of overexpressed genes were performed using Venn diagrams in GeneSpring GX. Lists of underexpressed genes were also compared using the same approach. All lists were compared in pairs and lists of genes in common between each pair were generated.
Generation of biological association networks
BANs were constructed with the aid of Pathway Architect software v3.0 (Stratagene-Agilent). Briefly, this software package generates interaction networks starting with the genes in a given list (entities) taking into account the information present in a database of known molecular interactions. The lists correspond to the collection of differentially expressed genes under specific conditions. The database of molecular interactions is composed of more than 1.6 million interactions divided into different classes (binding, regulation, promoter binding, transport, metabolism, protein metabolism and expression). The interactions are extracted from the literature using a Natural Language Processing (NLP) tool run on Medline abstracts (NLP references), plus those obtained from external curated databases like BIND (Biomolecular Interaction Network Database)  and MINT (Molecular INTeraction) . Interactions in the interaction database are scored according to five different categories: maximum, high, medium, low and minimal. Curated interactions (BIND and MINT sources) are given the maximum quality score, as are any interactions that have at least three NLP references. Pathway Architect gathers all that information to construct novel views as to how the entities in a list could be interacting with each other, even including entities not present in the original list (neighbors resulting from the expanded interaction). Customized analyses were performed to select relevant interaction networks with an associated high confidence index since such networks are likely to mirror biological significance. One-step expansion (using the expand network command) of the original set of entities with maximum score interaction were then analyzed by setting an advanced filter that included the categories of binding, expression, metabolism, promoter binding, protein modification and regulation. This procedure gives a final representation formed of a collection of nodes with different degrees of interrelationship. Some gene products from the original lists were not significantly connected with other members or neighbors and, therefore, were removed from the final view. Finally, members of the network were matched with expression levels.
Transfection of small interfering RNAs against selected genes
HT29 cells (30,000) were plated in 1 ml of -GHT medium and transfection was performed 18 hours later. For each well, Lipofectamine™ 2000 (Invitrogen, Carlsbad, CA, USA) in 100 μl of serum free -GHT medium was mixed in Eppendorf tubes with 100 nM of small interfering RNA (siRNA) in 100 μl of serum free -GHT medium. The mixture was incubated at room temperature for 20 minutes before addition to the cells. MTX (2 × 10-8 M) was added 48 hours after siRNA treatment and 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays  were performed 3 days after MTX addition. Treatment of MDA-MB-468 and MIA PaCa-2 cells was performed following the same protocol but using Metafectene™ (Biontex, Martinsried, Germany). A non-related siRNA was used as negative control; it was transfected in parallel with the other siRNAs and used to normalize the results.
Sense strand sequences of the siRNAs used
siRNA sequence (5'- 3')
Heat map generation
A global comparison of all cell lines was performed using GeneSpring GX v 7.3.1. The triplicate samples for each condition in each of the seven cell lines (42 samples) were imported into one single experiment. Normalization was performed in two steps: 'per chip normalization' (as described above) and 'per gene normalization', by which the samples were normalized against the median of all samples. Lists of genes displaying a false discovery rate-corrected P-value < 0.05 were generated for each cell line. As a filter, these values had to appear in at least two out of the seven cell lines. A hierarchical clustering method in GeneSpring was used to group genes on the basis of similar expression patterns over all samples. The distance matrix used was Pearson correlation, and average linkage was used as clustering algorithm. The same clustering method was used to group the cell lines on the basis of similar patterns of gene expression.
Gene mRNA levels were determined by real-time RT-PCR. Total RNA was extracted from cells using Ultraspec™ RNA reagent (Biotecx, Houston, TX, USA) following the recommendations of the manufacturer. For determining gene-node mRNA levels upon siRNA treatment, cells were treated as described above and total RNA was prepared 48 hours after transfection using the same reagent. In either case, complementary DNA was synthesized in a total volume of 20 μl by mixing 500 ng of total RNA and 125 ng of random hexamers (Roche, Mannheim, Germany) in the presence of 75 mM KCl, 3 mM MgCl2, 10 mM dithiothreitol, 20 units of RNasin (Promega, Madison, WI, USA), 0.5 mM dNTPs (AppliChem, Darmstadt, Germany), 200 units of M-MLV reverse transcriptase (Invitrogen) and 50 mM Tris-HCl buffer, pH 8.3. The reaction mixture was incubated at 37°C for 60 minutes. The cDNA product was used for subsequent real-time PCR amplification using an ABI Prism 7000 Sequence Detection System (Applied Biosystems, Foster City, CA, USA) with 25 ng of the cDNA mixture, the assays-on-demand from Applied Biosystems Hs00758822_s1 for DHFR and Hs00356991_m1 for Adenine Phosphoribosyltransferase (APRT) or the following primers: DKK1, 5'-AGTACTGCGCTAGTCCCACC-3' and 5'-CTGGAATACCCATCCAAGGTGC-3'; UGT1A, 5'-TAAGTGGCTACCCCAAAACG-3' and 5'-CTCCAGCTCCCTTAGTCTCC-3'; EEF1A1, 5'-CGTCATTGGACACGTAGATTCGGG-3' and 5'-GGAGCCCTTTCCCATCTCAGC-3'.
Gene copy number determination
Genomic DNA from either sensitive or resistant cells was obtained with the Wizard™ Genomic DNA Purification Kit (Promega) following the manufacturer's recommendations. One hundred nanograms of DNA and the assays-on-demand Hs00758822_s1 for DHFR and Hs99999901_s1 for 18S were used for real-time PCR amplification.
Preparation of total extracts for western blotting
Total extracts from cells, either sensitive or MTX-resistant, were used to assay DHFR protein levels. Cells were washed twice with ice-cold phosphate-buffered saline and scraped in 200 ml lysis buffer (50 mM Hepes, 500 mM NaCl, 1.5 M MgCl2, 1 mM EGTA, 10% glycerol (v/v), 1% Triton X-100 and protease inhibitor cocktail). Cells were incubated in ice for 1 hour with vortexing every 15 minutes and then centrifuged at 14,000 rpm at 4°C for 10 minutes. Five microliters of the extract were used to determine protein concentration by the Bradford assay (Bio-Rad, Munich, Germany). The extracts were frozen in liquid N2 and stored at -80°C. Fifty micrograms of both sensitive and resistant cell total extracts were resolved by 15% SDS-PAGE. Transference to PVDF membranes (Immobilon P, Millipore, Bedford, MA, USA) using a semidry electroblotter was followed by incubation with an antibody against DHFR (Davids Biotechnologie, Regensburg, Germany), and detection was accomplished using secondary horseradish peroxidase-conjugated antibody and enhanced chemiluminescence, as recommended by the manufacturer (Amersham/GE Healthcare, Buckinghamshire, UK). To normalize the results, blots were re-probed with an antibody against actin (Sigma, St. Louis, MO, USA).
Transfections, co-transfections and luciferase assays
HT29 cells, either sensitive or MTX-resistant, were seeded into six-well plates the day before transfection at a density of 2 × 105 cells/well in Ham's F12 medium containing 7% fetal bovine serum. Transfection was performed using FUGENE™ HD (Roche). For each well, 6 μl of FUGENE™ HD in 100 μl of serum-free medium was incubated at room temperature for 5 minutes. The mixture was added to 1 μg of TOPFLASH (Millipore) and incubated at room temperature for 20 minutes before addition to the cells. In co-transfections, 1 μg of TOPFLASH was mixed together with 2 μg of pBATEM2-CDH before the addition of FUGENE™ HD in serum-free medium. The total amount of DNA was kept constant at 3 μg, adding empty vector when necessary. Luciferase activity was assayed 30 hours after transfection.
In all cases, cell extracts were prepared by lysing the cells with 200 μl of freshly diluted 1× Reporter Lysis Buffer (Promega). The lysate was centrifugated at 13,000 g for 2 minutes to pellet the cell debris and the supernatants were transferred to a fresh tube. A 15-μl aliquot of the extract was added to 15 μl of the luciferase assay substrate (Promega) and the luminiscence of the samples was read immediately on a Gloomax 20/20 luminometer (Promega); light production (relative light units) was measured for 10 s. Each transfection was performed in triplicate. Protein concentration was determined by the Bradford assay and used to normalize the results.
Data are presented as mean ± standard error (SE). Statistical analyses were performed using the unpaired t-test option in GraphPad InStat version 3.1a for Macintosh (GraphPad Software). P-values < 0.05 were considered to be statistically significant.
Genes deregulated in methotrexate-resistant cancer cell lines
In a previous study, we analyzed the differential gene expression between sensitive and MTX-resistant cells derived from the human colon cancer cell line HT29 . In the present work we extend the study of gene expression profiles associated with MTX resistance by including another six MTX-resistant cell lines. Together, the studied cell lines represent colon cancer (CaCo2 and HT29), breast cancer (MCF-7 and MDA-MB-468), pancreatic cancer (MIA PaCa-2), erythroblastic leukemia (K562) and osteosarcoma (SaOs-2). Total RNA was extracted for the seven pairs of sensitive and MTX-resistant cell lines, and the expression profile of the 54,700 transcripts and variants included in the HG U133 PLUS 2.0 microarray from Affymetrix was compared between each pair using GeneSpring GX software v7.3.1. Upon normalization and statistical filtering of the data, lists of genes differentially expressed by at least twofold were built as described in Methods. These lists are presented as Additional data files 1 to 7. The data discussed in this report have been deposited in the Gene Expression Omnibus (GEO)  and are accessible through GEO series accession number [GSE16648].
Hierarchical clustering of genes and cell lines
Dihydrofolate reductase status in all cell lines studied
Validation of dhfr overexpression and copy number determination in the different cell lines
10.8 ± 0.7
16.1 ± 1.4
49.7 ± 1.1
83.4 ± 8.1
33.2 ± 0.7
58.1 ± 0.8
3.4 ± 0.1
0.9 ± 0.1
8.2 ± 1.1
32.2 ± 2.2
9.8 ± 0.2
1.9 ± 0.1
4.1 ± 1.1
0.6 ± 0.1
Identification of genes differentially expressed in common among different cell lines resistant to MTX
Genes differentially expressed in common among colon cancer cell lines resistant to MTX
FXYD domain containing ion transport regulator 3
Zinc finger, FYVE domain containing 16
Enolase 2 (gamma, neuronal)
Ca2+-dependent activator protein for secretion 2
Similar to annexin II receptor
Aldo-keto reductase family 1, member C2
Aldo-keto reductase family 1, member C2
Dickkopf homolog 1
Kelch repeat and BTB (POZ) domain containing 11
Zinc finger, FYVE domain containing 16
Aldo-keto reductase family 1, member C1
MutS homolog 3
Aldo-keto reductase family 1, member C1
MutS homolog 3
Aldehyde dehydrogenase 3 family, member A1
Solute carrier family 26 member 2
SWI/SNF related, regulator of chromatin a1
Transforming, acidic coiled-coil containing protein 1
Transforming, acidic coiled-coil containing protein 1
Disabled homolog 2
Aldo-keto reductase family 1, member B10
Myeloid/lymphoid or mixed-lineage leukemia
SH3 and PX domains 2A
Nucleoporin like 1
Transmembrane 4 superfamily member 12
Zinc finger protein 185 (LIM domain)
Membrane associated guanylate kinase
Platelet derived growth factor C
Syntrophin, beta 1
Apolipoprotein B, catalytic polypeptide-like 3G
Actin-related protein Arp11
Squamous cell carcinoma antigen
GLI pathogenesis-related 1 (glioma)
Response gene to complement 32
C-type lectin domain family 3, member A
Teratocarcinoma-derived growth factor 1/3
Retinoic acid receptor responder 2
Fibroblast growth factor 19
GLI pathogenesis-related 1
Interferon, gamma-inducible protein 16
Interferon, gamma-inducible protein 16
Genes differentially expressed in common among breast cancer cell lines resistant to MTX
UDP glucuronosyltransferase 1, polypeptides A1/3/4/5/6/7/8/9/10
UDP glucuronosyltransferase 1, polypeptides A1/4/6/8/9/10
UDP glucuronosyltransferase 1, polypeptides A1/4/6/8/9/10
UDP glucuronosyltransferase 1, polypeptides A1/3/4/5/6/7/8/9/10
UDP glucuronosyltransferase 1, polypeptides A1/3/4/5/6/7/8/9/10
Genes differentially expressed in common among colon cancer cell lines resistant to MTX
Ratio MIA PaCa-2
Phosphoinositide-3-kinase, regulatory subunit 3
Eukaryotic translation elongation factor 1 alpha 1
Single-stranded DNA binding protein 2
ATG10 autophagy related 10 homolog
Detection of nodes upon generation of biological association networks
Effect on MTX sensitivity of siRNAs designed against the mRNAs of node genes
Validation of DKK1, UGT1A and EEF1A1 overexpression in the resistant cells
5.66 ± 0.23
1.96 ± 0.03
23.41 ± 0.94
16.28 ± 0.19
3.89 ± 0.16
2.38 ± 0.47
1.85 ± 0.15
Given that the UGT1A family is involved in the metabolism of other drugs, we also performed combination treatments with siUGT1A and SN38, the active metabolite of the anticancer drug irinotecan. Transfection of the siRNA was performed as described above, and 1 nM SN38 was added 48 hours after siRNA treatment. These incubations led to a significant (P < 0.01) increase of 46% in SN38 sensitivity.
A non-related siRNA against the luciferase gene was used as a negative control in all experiments. Transfection of this siRNA was performed in parallel with the other siRNAs, and was used to normalize the results.
DKK1 is overexpressed in HT29 MTX-resistant cells due to higher activation of the Wnt pathway
The main objective of this work was to explore whether node genes could be identified from BANs constructed starting from genes differentially expressed in MTX-resistant cells from different human cancer cell lines, representative of five tissues. Those putative node genes may then be used as targets to increase the sensitivity toward MTX.
We started by determining and comparing the patterns of differential gene expression associated with MTX resistance in seven cell lines. The only differentially expressed gene in common among all the cell lines studied was dhfr. Its overexpression, at both the mRNA and protein levels, was confirmed in the MTX-resistant cells studied. The mRNA upregulation can be explained either by gene amplification of the dhfr locus, a well recognized mechanism for MTX resistance [15–17], or by an increase of dhfr transcription rate. In accordance with this, HT29, CaCo-2, MCF-7 and MIA PACA-2 resistant cells display an increased dhfr copy number. Indeed, amplification of the 5q14 locus, including dhfr and flanking genes, has been described in HT29 MTX-resistant cells . On the other hand, the other cell lines studied bear no changes in dhfr copy number. Thus, drug resistance in MDA-MB-468, K562 and Saos-2 cells may be caused by any of the other known mechanisms for MTX resistance [18–22]. Additionally, one has to take into account that MTX causes the differential expression of many genes that may be direct or indirect regulators of cell proliferation, survival or apoptosis, and that this expression pattern can contribute to modulation of the resistance phenotype. As described in this work, the overexpression of DKK1, UGT1A s or EEF1A1 could represent a mechanism, parallel to DHFR overexpression, that plays a role in MTX resistance; the possible contribution of the overexpression of each of these genes is addressed below. Other work has determined genes that correlate with the capability of parental cells to resist treatment using concentrations assessed to be clinically achievable in tumor tissue . Although this approach is different to the goal of our work, to identify genes differentially expressed in cells with acquired resistance to high concentrations of MTX, we compared the results obtained by Gyorffy et al.  with ours and found only six genes in common, namely CD99, CKMT1, DHRS2, IGFBP7, MAP7 and MYO1E.
Hierarchical clustering of all samples indicated that the MTX-resistant cells and their sensitive counterparts were highly correlated with regards to gene expression with each other. This is in accordance with reports showing that two breast tumor samples from the same patient before and after treatment with doxorubicin pair together in a hierarchical clustering . Moreover, the same authors proposed that the molecular program of a primary tumor could generally be retained in its metastases. Similar results were obtained with leukemic cells from patients .
Interestingly, as shown in Figure 1, the gene expression patterns for the cell lines from the same tissue origin were very similar. Sets of coordinately expressed genes provide gene expression signatures that can indicate where to find targets suitable for gene therapy. Thus, we generated a list of genes differentially expressed in common between the two colon cancer cell lines. This list included, among others, genes encoding DHFR, the target for MTX, three members of the AKR family and ENO2, which we have previously studied as modulators of MTX resistance [8, 13], and DKK1.
BAN construction using the genes differentially expressed in common between both colon cancer cell lines identified DKK1 as a highly interconnected node of the network, which could as such be a candidate druggable gene. DKK1 is a secreted protein involved in embryonic development  and is classically considered to function as an inhibitor of the canonical Wnt signaling pathway  (see  for a review). However, it does not take an active part in the Wnt/β-catenin pathway in colon cancer cells, as mutation of adenomatous polyposis coli (one of the components of Wnt pathway) occurs in most human colon cancers [29, 30], thus disconnecting the effector part of the signaling cascade from the Wnt receptors, where DKK1 exerts its inhibitory effect [31, 32]. This situation led us to hypothesize that DKK1 could have other cellular functions aside from its role in the Wnt pathway. Indeed, a role for DKK1 overexpression in cancer , including hepatobastomas  and breast cancer bone metastasis , aggressive tumors, epithelial-mesenchymal transition  and proliferation , has been previously suggested, although its precise mechanism of action has not yet been elucidated. In the case of HT29 MTX-resistant cells, the role of DKK1 is unclear, although it seems to be related to the resistant phenotype, since treatment with the siRNA against DKK1 mRNA showed a chemosensitization toward MTX. In keeping with this, Katula and collaborators  showed that folate deficiency led to the downregulation of DKK1, and that MTX inhibited DKK1 transcription. Thus, DKK1 overexpression in HT29 MTX-resistant cells could constitute a mechanism to overcome the transcriptional repression exerted by MTX.
It is worth noting that we had previously proposed the activation of the Wnt/β-catenin pathway to be an important step in MTX resistance in HT29 colon cancer cells . In this cell line, E-cadherin is chromosomically lost and underexpressed, thus allowing β-catenin to play its function in gene transcription. In order to shed some light on the possible role of the Wnt pathway in the overexpression of DKK1 in HT29 resistant cells, we performed transient transfection experiments with a luciferase reporter of β-catenin-mediated transcriptional activation. These experiments showed that the Wnt pathway was more active in the HT29 resistant cells than in HT29 sensitive cells, and that re-expression of E-cadherin in the resistant cells resulted in lower β-catenin-mediated transcriptional activation, probably due to recruitment of β-catenin to the adherent junctions. Interestingly, DKK1 is transcriptionally regulated by Wnt/β-catenin signaling. Thus, constitutive activation of this signaling pathway through β-catenin, downstream of adenomatous polyposis coli, could represent a mechanism for DKK1 overexpression in HT29 MTX-resistant cells.
UGT1A s were the only genes differentially expressed in common between both breast cancer cell lines analyzed. UGTs comprise a family of membrane glycoproteins that come from one single gene located on chromosome 2q37, rendering nine functional UGT1A proteins by alternative splicing of 13 different first exons with the common exons 2 to 5 . UGTs are involved in phase II metabolism of a wide range of metabolites, both endogenous and exogenous . Glucuronidation is an important metabolic process, as it carries out the biotransformation of lipophilic substrates into hydrophilic glucuronides, which can be more easily removed from the body. Among the different products that can undergo glucuronidation, we find analgesics, sex hormones, flavonoids, rifampicin, bilirubin and tobacco-specific carcinogens [40, 41]. Some anticancer drugs, such as topotecan, irinotecan, SN-38 (the active metabolite of irinotecan), doxorubicin and 4-hydroxytamoxifen, have also been described to be substrates of UGT1A activity [42–47]. Metabolism by UGT1A family members has been described to induce resistance toward daunorubicin (in both cell lines and rat hyperplastic liver nodules) [48, 49], mycophenolic acid [50, 51], mitoxantrone , SN-38 [53, 54], camptothecin  and other drugs [56, 57]. Indeed, drug inactivation by metabolism within tumor cells is recognized as an important mechanism of drug resistance, and, specifically, glucuronidation by UGT enzymes has been proposed to contribute to multidrug resistance of several chemotherapeutic drugs . From our results using a siRNA against UGT1A mRNA we can point out a role for this family of genes in MTX resistance in MDA-MB-468 breast cancer cells. In fact, Hanioka et al.  reported that β-naphthoflavone induced UGT1A mRNA levels. This could be a feasible explanation for the UGT1A overexpression we observed in the breast cancer MTX-resistant cells used in our study.
EEF1A1 was overexpressed in common among MIA PaCa-2, K562 and Saos-2 resistant cell lines. EEF1A1 is a ubiquitously expressed protein elongation factor that recruits amino-acetylated tRNAs to the A site of the ribosome (see  for a review). Although it has been traditionally described as a cellular housekeeper enzyme, overexpression of EEF1A1 is found in melanomas and tumors of the pancreas, breast, lung, prostate and colon [59, 60]. It has been demonstrated that EEF1A expression is related to increased cell proliferation [61, 62], oncogenic transformation , delayed cell senescence  and metastasis . Moreover, increased EEF1A1 expression has been related to cisplatin , doxorubicin  and MTX resistance , maybe due to its ability to inhibit apoptosis . It has been proposed that EEF1A overexpression promotes cell growth and replication by contributing to an overall increase in protein translation. Antisense-mediated abrogation of EEF1A1 expression inhibits tumorigenesis and anchorage-independent cell replication in prostate tumor cells . Our functional analyses using siRNA technology against EEF1A1 are in keeping with these results, and show a chemosensitization of MIA PaCa-2 cells, thus stating a role for EEF1A1 in MTX resistance in this cell line.
In summary, our results provide evidence that node genes can be identified by constructing BANs with lists of genes differentially expressed in common between cell lines resistant to MTX. RNA interference technology has enabled us to demonstrate a role for DKK1, UGT1As and EEF1A1 in MTX resistance.
BANs were constructed using genes differentially expressed in common between cells resistant to MTX from seven human cancer cell lines representative of five tissues. We have been able to identify important node genes in the BANs, namely DKK1 in colon cancer cells, UGT1A s in breast cancer cells and EEF1A1 in pancreatic cancer, leukemia and osteosarcoma cells. These three genes were functionally validated using siRNAs against their respective mRNAs, which resulted in increased sensitivity to MTX.
Additional data files
The following additional data are available with the online version of this paper: a table listing of genes differentially expressed at least twofold in HT29 MTX-resistant cells (Additional data file 1); a table listing genes differentially expressed at least twofold in Caco-2 MTX-resistant cells (Additional data file 2); a table listing genes differentially expressed at least twofold in MCF-7 MTX-resistant cells (Additional data file 3); a table listing genes differentially expressed at least twofold in MDA-MD-468 MTX-resistant cells (Additional data file 4); a table listing genes differentially expressed at least twofold in MIA PaCa-2 MTX-resistant cells (Additional data file 5); a table listing genes differentially expressed at least twofold in K562 MTX-resistant cells (Additional data file 6); a table listing genes differentially expressed at least twofold in Saos-2 MTX-resistant cells (Additional data file 7).
biological association network
Dikkopf homolog 1
eukaryotic translation elongation factor 1 alpha 1
Natural Language Processing
small interfering RNA
UDP glucuronosyl transferase.
This work was supported by grants SAF05-247 and SAF08-00043 from 'Plan Nacional de I+D+I', and ISCIII-RETIC RD06/0020. Our research group holds the 'quality distinction' from the 'Generalitat de Catalunya' SGR09-118. ES was a recipient of a fellowship from the 'Ministerio de Ciencia y Tecnología' (MCYT), CA is a recipient of a fellowship from the 'Generalitat de Catalunya', and CO is a recipient of a fellowship from 'Federación Española del Café'. pBATEM2-CDH was a kind gift of Dr Duñach, Universitat Autonoma de Barcelona, Spain.
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